An ordinary flea has a mass of 210. How many newtons does it weigh?

The flea is quite small for its mass to be expressed in grams. I assume that the given mass is in milligrams. Convert this value first to kilograms.

(210 mg) x (1 g / 1000 mg) x (1 kg / 1000 kg) = 2.1x10^-4 kg

Multiply this value by the Earth's acceleration due to gravity, 9.8 m/s².

(2.1x10^-4 kg) (9.8 m/s²) = 2.058 x 10^-3 N

F=ma

is the formula you need to find the amount of newtons force.

If it weighs 210, and it's being pulled down by gravity at 9.8m/s^2, all you have to do is multiply the two numbers to get 2058.